We are required to have a security person at the door when the building we daven at is otherwise closed (on Shabbat). For financial reasons, we use rotating volunteers, but sometimes, like for Mincha, we have only ten men. The security person can just barely see into the shul room from the back door of the room where we daven. The last person in the room can barely see them. Can that security person be the tenth, bedieved or lekhatchilah?